1. Field of the Invention
This invention relates to a novel copolymer comprising sulfur dioxide and at least one vinyl compound. More particularly it relates to a novel resist material of a positive type to be decomposed by electron beam or X-rays with a high sensitivity, and having a superior durability to dry etching, and the above-mentioned copolymer used as an active ingredient for the above-mentioned resist material, and further a process for producing the same.
2. Description of the Related Art
Advance of LSI (large scale integrated circuitry) in several recent years is remarkable and a high performance has been required for resist materials used for lithography of IC. The high performance referred to herein means that the materials are highly sensitive to light, electron beam and X-rays and have a high resolution degree. More recently, in the field of IC technique, a dry etching process has come to occupy the main current in place of conventional wet etching, and thus it has come to be required for resist materials to have a superior durability to dry etching.
In general, plasma etching is superior in fine resolution degree, reproducibility and operability to chemical etching, and further, has an advantage that stains formed in the use of chemical etchants or handling problem do not occur.
Now, as conventional high molecular materials for positive type electron beam resists, polyolefinsulfone, polymethyl methacrylate, etc. have been known ("Precision fine processing in electronics" edited by Seii Naraoka, and issued by Sogodenshi Shuppansha, 1983). However, many positive type resist materials cannot endure plasma discharge. Thus, chemical etching has generally been employed in spite of the above-mentioned drawbacks.
Thus, development of a positive type resist having a superior plasma etching durability has been urgently desired.
Now, resist materials having a superior resistance to oxygen plasma etching can be obtained by preparing Si-containing polymers.
The reason has been said to consist in that resist films obtained from Si atoms-containing polymers degrade by means of oxygen plasma to form a thin metal oxide layer on the surface of the film, which layer constitutes a barrier layer against the oxygen plasma ((Silicone resist) "Electron materials", edited by Kei Murase, July, 1985).
In another fact, it has been known that resists having an aromatic ring in the structure thereof have a high resistance to dry etching. For example, as an electron beam negative resist for dry etching, polystyrene, polychloromethylstyrene, etc. have been known.
As an electron beam positive resist, poly((styrene).sub.p -sulfone) has been known. This resist has a high resistance to dry etching, but its sensitivity is low.